3.4 Black holes

There have been very few observations of black hole binaries in globular clusters. Although there have
been hints of possible black hole binaries in extragalactic globular cluster systems [6, 49], there are no
known black hole binaries in the galactic globular cluster system. All of the globular cluster high luminosity
LMXBs exhibit the X-ray variability that is indicative of nuclear burning on the surface of a neutron star. It
is possible that some of the recently discovered low luminosity LMXBs may house black holes instead
of neutron stars [234], it is more likely that they are simply unusual neutron star LMXBs in
quiesence [240]. Finally, there is very circumstantial evidence for the possible existence of an intermediate
mass black hole (IMBH) binary in NGC 6752 based upon an analysis of the MSP binary PSR
A [34, 35].